In the known rotary tube furnace plants, the zone through which the exhaust gases pass and which is provided with rotary lifting members comprises a ring disposed adjacent the material inlet end of the furnace which rotates jointly with the furnace. The material extracted from the second stage (i.e., the second from the bottom) of the cyclone pre-heater is fed into this ring. As this ring, which forms a component of the rotary tube furnace, is rotating, this material is fed into the exhaust gases passing through the ring. The material together with the furnace exhaust gases then passes into the first (lowermost) stage of the cyclone pre-heater, whence after separation it is fed into the rotary tube furnace.
While such a construction does allow the length of the rotary furnace to be reduced in comparison with other known constructions (not having the ring described), it does on the other end have considerable disadvantages.
Because of the reduced length of the furnace, the exhaust gases in the vicinity of the said ring or material inlet are still at relatively high temperature. This high gas temperature, which is dangerous for the following parts of the plant, especially for pipes and chutes of narrow cross-section, is during normal operation of course brought down to a sufficiently low value by the material fed through the ring into the gas stream. But if for any reason during operation the feed of material briefly drops, temperatures at the inlet zone of the furnace rise rapidly and lead to dangerous overheating. If the supply is then restored a little later, unwanted accumulations often occur on the walls of gas pipes, especially in the transition area between pre-heater and rotary furnace.